1. Field of the Invention
This invention relates to a system for the reduction of harmful exhaust emissions from diesel engines, and more particularly to a system for increasing the effectiveness of the oxidation of the oxidizable components in the exhaust emissions.
2. Description of Related Art
Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (xe2x80x9cCOxe2x80x9d), unburned hydrocarbons (xe2x80x9cHCxe2x80x9d) and nitrogen oxides (xe2x80x9cNOxxe2x80x9d), but also condensed phase materials (liquids and solids) which constitute the so-called particulates or particulate matter (xe2x80x9cPMxe2x80x9d). The total particulate matter (xe2x80x9cTPMxe2x80x9d) emissions are comprised of three main components. One component is the solid, dry, solid carbonaceous fraction or soot. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the TPM is the soluble organic fraction (xe2x80x9cSOFxe2x80x9d). The soluble organic fraction is sometimes referred to as the volatile organic fraction (xe2x80x9cVOFxe2x80x9d), which terminology will be used herein. The VOF may exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust, and are generally present as condensed liquids at the standard particulate collection temperature of 52xc2x0 C. in diluted exhaust, as prescribed by a standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure, discussed further below. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel.
The third component of the particulates is the so-called sulfate fraction. Diesel fuel contains sulfur, and even the low sulfur fuel available in the U.S. may contain 0.05% sulfur. Upon combustion of the fuel in the engine, nearly all of the sulfur is oxidized to sulfur dioxide which exits with the exhaust in the gas phase. However, a small portion of the sulfur, perhaps 2-5%, is oxidized further to SO3, which in turn combines rapidly with water in the exhaust to form sulfuric acid which collects as a condensed phase with the particulates as an aerosol, or is adsorbed onto the other particulate components, and thereby adds to the mass of TPM.
Emissions from diesel engines have been under increasing scrutiny in recent years and standards, especially for particulate emissions, have become stricter. In 1994 the particulate emission standards in the U.S. for new engines allowed no more than a total of 0.1 grams per brake horse power hour (g/BHP-h). For diesel engines in buses operating in congested urban areas the particulate emissions standard was even stricter, 0.07 g/BHP-h TPM. Both of these standards were seen as significant reductions relative to the prior particulate emission standard of 0.25 g/BHP-h which had been in effect since 1991. Starting in 1994, for the first time, engine technology developments alone were found to be incapable of meeting the new standards, and for some engines aftertreatment technology, for example, diesel oxidation catalyst (DOC) units, as discussed further below, were necessary.
Current engines are generally capable of meeting the 1994 NOx emissions standards of 5.0 g/BHP-h, but by only a slim margin. Diesel engines, because they operate with a great excess of combustion air (lean exhaust) typically have emissions of CO and gas phase HC""s which are well below the 1994 emissions standards of 15.5 g/BHP-h and 1.3 g/BHP-h, respectively. Therefore, the key emission control concerns for diesel engines now and for the immediate future are the reduction in particulates (TPM) and NOx emissions.
Emissions of NOx from diesel engines can be reduced by retarding injection timing. However, this is accompanied by a corresponding increase in particulate emissions, particularly of the dry carbon or soot portion. Emissions of NOx can also be reduced by applying exhaust gas recirculation (EGR) technology. However, this is also accompanied by a corresponding increase in particulate emissions. Thus, both of these engine technologies are constrained by a trade-off or balance between TPM and NOx emissions.
Additional EPA requirements went into effect at the beginning of 1995 which apply to urban buses equipped with engines manufactured prior to 1994. These requirements apply to engines in service when they come due for rebuilding. Following engine rebuild the requirements must be met. One portion of these requirements specifies that if technology can be demonstrated for particulate reduction for these pre-1994 bus engines, that technology would be mandated for use on those engines for which it is certified. Two tiers of such technology/emissions reduction attainment were promulgated including:
1. Meet the 1994 Emissions Standard of 0.1 g/BHP-h TPM, with a technology cost cap of about $8,000.
2. Reduce Engine-Out TPM Emissions by at least 25%, with a technology cost cap of about $3,000.
The first of the above attainment levels, which is considered the stricter of the two requirements, if demonstrated and certified, takes precedence. Thus, the 25% TPM reduction tier was considered a xe2x80x9cfall-backxe2x80x9d position, if the 0.1 g/BHP-h TPM tier could not be met. It is clear from the strict emissions requirements for new diesel engines used in urban buses and the attainment requirement for pre-1994 bus engines that a major challenge exists for this type of application.
Diesel engines used in urban bus applications in the U.S. are of many types, both two-cycle and four-cycle, supplied by a range of engine original equipment manufacturers (OEM""s). However, a large percentage of urban transit buses have two-cycle engines from one manufacturer (Detroit Diesel Corp.). The emissions reduction system of this invention is considered to be applicable to any diesel engine for lowering emissions and the level of emissions reduction attained is expected to be dependent on the specific engine, its operating parameters and baseline engine-out emissions. However, this invention has been found to be especially useful for two-stroke diesel engines, and as demonstrated herein, can be used with such engines manufactured prior to 1994 to bring them into compliance with the 1994 particulate emissions standard of 0.1 g/BHP-h TPM, as discussed above.
Oxidation catalysts comprising a platinum group metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both HC and CO gaseous pollutants and particulates, i.e., soot particles, by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been contained in units called diesel oxidation catalysts (DOC""s), or more simply catalytic converters or catalyzers, which are placed in the exhaust train of diesel power systems to treat the exhaust before it vents to the atmosphere. However, by the time the exhaust gas reaches the catalyzer, it has generally lost a considerable amount of heat, both by radiation through the engine and exhaust system walls, and by intentional power transfer at the turbocharger. Because the efficiency of such catalytic oxidation processes is generally a direct function of the gas temperature, such temperature losses can have a significant negative impact on the effectiveness of the catalyzer.
One approach to improving the effectiveness of the catalyzer is to maintain the exhaust temperature at as high a level as possible, from the combustion chamber and through the connecting exhaust train to the catalyzer. Heat-insulating structures and heat-insulating coatings, i.e., thermal barrier coatings have been employed by those skilled in the art to enhance the thermal efficiency of internal combustion engines by permitting more complete fuel burning at higher temperatures. Typically, such heat-insulating coatings have been applied to all of the chamber surfaces, including the cylinder walls and head and piston combustion faces to prevent heat loss. Heat-insulating structures and heat-insulating coatings have also been used in automobile exhaust systems to maintain high exhaust temperatures required by thermal reactors and catalytic converters, thus reducing the emission of unburned hydrocarbons emitted into the atmosphere as an undesirable component of exhaust gas.
U.S. Pat. No. 5,384,200 is directed to particular thermal barrier coatings and methods of depositing such coatings on the surfaces of combustion chamber components. As discussed in that patent, insulating the combustion chamber components reduces the amount of heat loss in the engine. The higher temperature in the combustion chamber results in a more complete combustion of the fuel in the chamber, and also results in a hotter exhaust being delivered to any downstream catalytic converters to promote more effective oxidation of the oxidizable components of the exhaust stream.
The use of thermal barrier coatings has also been suggested for engine components other than in-cylinder surfaces. In a paper entitled xe2x80x9cHigh Performance Coatings for Diesels and Other Heat Enginesxe2x80x9d, by Roy Kamo, presented at the Thermal Spray Coatings Conference, Gorham Advanced Materials Institute, Orlando, Fla., on Sep. 12-14, 1993, it is suggested that engine performance can be improved by applying thermal barrier coatings to various engine components. In addition to in-cylinder surfaces such as the piston crown and cylinder head, the article also suggests the exhaust port, exhaust manifold and turbocharger housing.
A typical diesel power system includes a diesel engine and an exhaust train through which the exhaust from the diesel engine passes. The present invention is directed to methods and apparatus for reducing the total particulate matter emissions in said exhaust from the diesel engine. One embodiment of the method of the present invention comprises thermally insulating at least a portion of the surface of the exhaust train which comes into contact with the exhaust with a thermal barrier coating, and incorporating an oxidation catalyst into at least a portion of the thermal barrier coating in operative contact with the exhaust. This is accomplished by thermally insulating at least a portion, preferably the hottest portion of the exhaust train which carries the hot exhaust gas stream from the diesel engine to the atmospheric vent. The insulation is applied to surfaces of the exhaust train which are in direct contact with exhaust gas, that is, the inside surfaces of the exhaust train components. The oxidation catalyst is incorporated into at least a portion of the thermal coating, and optionally into substantially all of the thermal coating.
Preferred oxidation catalysts for use in the present invention are base metal oxides, particularly the rare-earth metal oxides, or mixtures of materials containing the base metal oxides or rare-earth metal oxides. Preferred rare-earth oxide catalysts for use in this invention are praseodymium oxide and ceria. Good results are also obtained with other rare-earth oxides, as discussed further below. The base metal oxides can be used alone, or in combination with catalytic platinum group metals, such as platinum, palladium and rhodium.
By insulating the exhaust train in accordance with the present invention, the effectiveness of the oxidation of the oxidizable components of the exhaust is increased in a downstream diesel oxidation catalyst (DOC) unit, and this decreases the level of undesirable emissions in the exhaust. Incorporating an oxidation catalyst into the thermal coating further reduces the emissions, particularly the TPM emissions. The catalyzed thermal barrier and the downstream DOC unit combine for a significant reduction in the overall pollution level in the exhaust.
Typically, the exhaust train of a diesel power system includes a manifold to collect the exhaust from the engine and channel it into one or more exhaust pipes. Being closest to the engine, the manifold is generally the hottest section of the exhaust train. Therefore, in a preferred embodiment of the present invention, at least a portion of the inner surface of the manifold is insulated to reduce the amount of heat lost through the manifold walls and thus maintain the exhaust at high temperature. Preferably, substantially the entire inner surface of the manifold is coated, that is at least about 90% of the area exposed to the hot exhaust gases.
From the manifold, pipes carry the exhaust through various apparatus which may be present in the exhaust train. Typically, a turbocharger is provided downstream of the manifold. Such devices are well known to those skilled in the art. A turbocharger mechanically extracts power from the exhaust stream, such as by a compressor driven by the exhaust, and transfers it to the inlet air stream to improve the overall efficiency of the diesel power system. As a result of such power extraction, the temperature of the exhaust gas generally drops significantly, such as about 100xc2x0 F. or more, as it passes through the turbocharger. It is therefore a further preferred embodiment of the present invention not only to insulate the manifold, but also to insulate the pipe or pipes connecting the manifold to the turbocharger, when a turbocharger is present.
After the exhaust exits the turbocharger, it is at a lower temperature. Downstream of the turbocharger, many commercial diesel power systems include a diesel exhaust oxidizer (DOC), as discussed above, for oxidizing the oxidizable components of the exhaust stream. Generally, the hotter the exhaust is when it enters the catalytic oxidizer, the more effective the oxidizer is in oxidizing the harmful oxidizable components. It is therefore a further embodiment of the present invention to insulate the pipes connecting the turbocharger to the downstream catalytic oxidizer.
A further embodiment of the present invention is to combine the insulative coating of the exhaust train, as discussed above, with insulative coating of the surfaces of the combustion chamber components, in order to maximize the combustion of the fuel in the combustion chamber and to further impede heat loss from the exhaust stream. The surfaces to be coated can include the piston crown, the cylinder head and the valve faces, as well as any other surfaces which are exposed to the combustion.
As discussed above, catalytic converters in diesel power systems are located in the exhaust train, and the effectiveness of the catalysts in such converters is reduced by temperature loss in the exhaust train. In accordance with one aspect of the present invention the catalytic oxidation of oxidizable components in the exhaust stream is improved by providing catalysts in the thermal barrier coatings which are applied to the exhaust train. By providing the catalysts in the high temperature end of the exhaust train, the catalysts are able to act on the exhaust gas when it is at its highest temperature. Furthermore, because such oxidation is an exothermic reaction, it is possible that this catalytic oxidation may increase the temperature of the exhaust gas, thus promoting more effective oxidation downstream at the catalytic converter.
In another embodiment of the present invention, in which the diesel power system exhaust train includes a turbocharger, the method of reducing the total particulate matter emissions in the exhaust simply comprises providing an oxidation catalyst in the exhaust train between the engine and the turbocharger. In this case, the oxidation catalyst can be mounted on the operating surfaces of a monolithic support, of the type well known in the art. As discussed further below, the turbocharger can significantly reduce the temperature of the exhaust gases. By providing catalyst in the exhaust train prior to the turbocharger, the catalytic oxidation can be conducted at the elevated exhaust temperatures before the turbocharger is reached. Optionally, the inner surfaces of the exhaust train can also be insulated, as discussed above. Also, if the surfaces are insulated, additional catalyst can be incorporated into a portion or substantially all of the insulation.
In a further embodiment of the present invention, when the combustion chamber is provided with a thermal barrier coating, an oxidation catalyst is provided on or in such coating in the combustion chamber. Oxidation catalysts in the combustion chamber can promote more complete oxidation of the fuel, thereby decreasing the amount of undesirable emissions sent to the exhaust train.
In a particular embodiment of the present invention, catalytic ceramic coatings are applied to the in-cylinder surfaces of the combustion chambers of an internal combustion engine, especially a compression ignition (diesel) engine. These coatings are of low thermal conductivity compared with standard metal parts, and provide a thermal barrier at the combustion chamber walls which keeps heat in the cylinder and promotes more complete combustion of the fuel, thereby giving greater fuel efficiency and lower emissions of particulates (SOF and dry-carbon/soot). In addition the coatings are provided with catalytic surfaces which further promote combustion of unburned fuel and particulates for increased efficiency and lower particulate emissions.
Another aspect of the present invention is that it was found that providing smooth and non-porous surface properties to the coatings, also contributes to improved combustion of unburned fuel and soot. This is believed to be the result of low drag at the surface which allows the swirl and mixing of the fuel. Such smooth and non-porous surfaces also reduce adsorption of the fuel which impacts the coated combustion chamber walls. Such adsorption can cause slow and incomplete combustion. The smoothness of thermal sprayed coated surfaces can be further improved by sanding or polishing. The increased smoothness can further reduce drag at the surface and thereby improve the mixing and swirling action of the fuel-air mixture, which in turn leads to better combustion.
In a particular embodiment of the present invention a top coat of mullite is used to protect the ceramic coating. The ceramic coatings are essentially the same as otherwise described herein for insulative coatings, except that a top layer of an alumino-silicate (mullite) ceramic layer is provided. The layer should be at least about 2 mils thick, and preferably about 3 to 5 mils thick. Under this typically is the yttria-stabilized zirconia layer and beneath that the bond coat. These coatings are therefore three layer coatings with an overall thickness approximately the same as the above coatings without mullite. The reason for the mullite top layer is to provide a very chemically inert ceramic surface with high resistance to the corrosive and aggressive materials encountered in the combustion and exhaust (sulfur and sulfates, calcium, zinc, nitrogen oxides and nitrates, chlorides, phosphorus, etc.). The xe2x80x9cas sprayedxe2x80x9d mullite top layer is less porous (about 3% porosity) than the zirconia layer, which is about 10% porosity.
The combustion chamber surfaces which are coated with the catalytic materials, such as catalytic oxides, will be at least the crowns (including the bowls of pistons which have bowls for receiving injected fuel, as is well-known in the art). The exhaust valves and cylinder head fire decks can also be coated with the catalytic oxides. The overall coating thickness should be relatively thin, for example, less than about 20 mils. and preferably less than about 15 mils. These thicknesses are the total of all coatings, such as a bond coat, ceramic thermal barrier layer(s) and catalytic oxide layer(s). This thickness criteria is to produce a coating with good thermo-mechanical properties and which will exhibit good long-term durability in-cylinder.
Another aspect of this invention is the use of stainless steel bond coats, particularly martensitic steels, to give good durability when the coatings are applied to aluminum alloy surfaces. Aluminum alloy pistons are used in many internal combustion engines, thus the need for the special bond coat. The typical alloy bond coats used with prior thermal spray coatings have been comprised of MCrAlY alloys where M=Ni, Co, Fe, etc., as is well-known in the art. In accordance with the present invention, stainless steel, particularly martensitic stainless steel, such as type 431 SS, has been found to be particularly useful as a bond coat to bond ceramics, such as yttria stabilized zirconia, to aluminum substrates.
It is believed that the reason such stainless steels are effective as bond coats is related to the relative coefficients of thermal expansion (CTE) of the components. Aluminum has a CTE of about 23.0, and aluminum alloys have CTE""s in the general range of about 21 to 25. The ceramic thermal barrier coatings have CTE""s of roughly about 6 to 8, with standard 7% yttria stabilized zirconia being about 7.6. The stainless steels used in the present invention, particularly martensitic stainless steels, have relatively low CTE""s, which approximate those of the ceramic thermal barrier coatings. Such steels are commonly referred to as being xe2x80x9clow-shrinkxe2x80x9d, because of their relatively low CTE. At the same time, such stainless steels also bond well to the aluminum substrate. It is believed that the use of a stainless steel which has a coefficient of expansion relatively close to that of the ceramic coatings acts to keep the ceramic firmly adhered to the aluminum. The CTE of type 431 SS martensitic stainless steel is about 6.6, which is even less than that of the yttria stabilized zirconia used as the thermal barrier coating in some of the examples below. The stainless steel bond coat does not move differentially to the zirconia ceramic, and remains firmly anchored to the aluminum substrate. As a result, the tendency for the coating to crack and spall off such aluminum substrates is greatly reduced
A further aspect of the present invention is providing an improved diesel oxidation catalyst (DOC) unit in the exhaust train. The improved DOC unit more effectively oxidizes the oxidizable components of the exhaust stream, thus reducing the TPM level of the exhaust. The improved DOC unit comprises a metal monolithic catalyst support rather than a ceramic support as used in other units.